Field of the Invention
The present invention relates to a field effect transistor.
Background Art
A GaN high-electron-mobility-transistor (HEMT) device is formed by using a substrate having a GaN-based epitaxial layer formed on a Si substrate. With the GaN HEMT device, high-efficiency, high-power-density, high-voltage operation can be realized in comparison with operation with a Si- or GaAs-based device because of its excellent material properties including a high saturated electron velocity and a high dielectric breakdown withstand voltage. This enables a transmitter to be reduced in size, weight and power consumption. Therefore, GaN HEMT devices are considered promising as a next-generation radiofrequency high-output electron device for communication. For example, on the market of portable telephone base station amplifiers as a typical application field, where Si-LDMOS devices are widely available presently, a device capable of operating with a further increased output and efficiency has been in demand with the increases in communication frequencies and the development in communication technology, and there are increasing expectations for GaN HEMT devices.
While there are expectations for GaN HEMT devices in terms of performance, there is a product price problem with making GaN HEMT devices practically available on the market. Diffusion of GaN HEMT devices has already been started in various market fields. On the market of portable telephone base station amplifiers, however, the state of GaN HEMT devices being high-priced in comparison with that of Si-LDMOS devices widely available on the market is thought to be one of the hindrances to the promotion of the diffusion.
GaN HEMT devices presently used for communication ordinarily use a substrate having a GaN-based epitaxial layer grown on a SiC substrate. Because the SiC substrate is much higher in price and smaller in substrate diameter than the Si substrate, the price of the GaN HEMT device finished as a product is necessarily increased.
On the other hand, Si substrates which can be easily increased in diameter and are low-priced in comparison with SiC substrates are also used as a substrate material for GaN HEMT devices. With GaN HEMT devices using Si substrates, there are expectations of producing the devices at a manufacturing cost close to that of manufacturing of Si-LDMOS devices as well as expectations of overcoming the above-mentioned product price problem.
GaN HEMT devices using Si substrates as their substrates, however, entail a problem in terms of device characteristic described below. In general, it is desirable that a radiofrequency output electronic device use, as a substrate material for achieving high performance at radiofrequencies, a substrate material having a high specific resistance for the purpose of reducing a parasitic capacitance. In a case where a Si substrate is used as a substrate for such a device, therefore, a high-resistance substrate of 1000 Ω·cm or higher made by a floating zone (FZ) method which enables easily making a substrate of a higher resistance in comparison with the Czochralski (CZ) method, the mainstream among Si crystal growth methods, is ordinarily used. A GaN HEMT device made by using a high-resistance Si substrate of 1000 Ω·cm or higher can actually have electrical characteristics which, at least in the current frequency band in which the device is used as a portable telephone base station amplifier, compare favorably with those of a GaN HEMT device made by using a SiC substrate as its substrate.
A semiconductor exhibits a material characteristic such that the intrinsic carrier density increases with increase in temperature. Because Si is a material narrower in bandgap than SiC, the intrinsic carrier density of Si is higher than that of SiC. Further, the temperature dependence of the intrinsic carrier density of Si is increased if the specific resistance at room temperature is increased. Because of the material characteristic of such a high-resistance Si substrate, a GaN HEMT device using the high-resistance Si substrate has a problem that if the temperature during RF operation is increased, the specific resistance of the Si substrate is reduced to cause an increase in loss and a considerable reduction in gain and hence reductions in output and efficiency.
As means for solving the problem of degradations in RF characteristics during high-temperature RF operation in a GaN HEMT device using a Si substrate, each of increasing the spacing between gate regions which are self-heat generation sources, increasing the so-called gate pitch and reducing the parasitic capacitance produced between drain electrodes and the substrate by reducing the drain electrode width is effective. More specifically, the thermal resistance of the device can be reduced by increasing the gate pitch, thus enabling limiting of the increase in temperature during RF operation and hence limiting of the increase in intrinsic carrier density. The drain electrode area can be reduced by reducing the drain electrode width, thus enabling reducing the parasitic capacitance produced between the drain electrodes and the substrate. As described above, deteriorations in RF characteristics during high-temperature RF operation in a GaN HEMT device using a Si substrate can be effectively limited if the two measures can be simultaneously taken.
On the other hand, a semiconductor device in which a drain electrode is divided into two or more parts such that the sum of the widths of the divided parts of each drain electrode is smaller than the width of one source electrode has been proposed (see, for example, FIG. 4 in National Publication of International Patent Application No. 2008-518462). In this way, the gate pitch can be designed independently of the drain electrode width, thus enabling limiting the thermal resistance. Further, even when the gate pitch is increased, the drain electrode area can be reduced by dividing the drain electrode into two or more parts, thus avoiding an increase in parasitic capacitance. This leads to limiting of deteriorations in RF characteristics during high-temperature RF operation in a GaN HEMT device using a Si substrate.
A GaN HEMT device, however, has a high-concentration two-dimensional electron gas layer (2DEG) at an AlGaN/GaN hetero interface, and the influence of the parasitic capacitance between a channel layer and the substrate in drain regions between two or more divided parts of a drain electrode is considerably large. Therefore, the parasitic capacitance reduction effect is restricted if only a reduction in drain electrode area is made. As a result, the effect of limiting deteriorations in RF characteristics during high-temperature RF operation in a GaN HEMT device using a Si substrate is restricted.